Degree Of Nitridation Research Articles

Studies of Phase Transformation Kinetics in the System of Nanocrystalline Iron/Ammonia/Hydrogen at the Temperature of 350 °C by Means of Magnetic Permeability In Situ Measurement

The kinetics of phase transformations in the nitriding process α-Fe → γ’-Fe4N → ε-Fe3-2N of the pre-reduced iron ammonia synthesis catalyst was investigated under in situ conditions (atmospheric pressure, 350 °C) by measuring changes of mass, gas phase composition, and magnetic permeability in a differential tubular reactor. The iron nanocrystallite size distribution according to their specific active surface areas was measured, and it was found that the catalyst is bimodal as the sum of two Gaussian distributions, also differing in the value of the relative magnetic permeability. Relative magnetic permeability of small α-Fe crystals in relation to large crystals is higher by 0.02. In the area of α → γ’ transformation, the magnetic permeability dependencies change, proving the existence of two mechanisms of the α-Fe structure change in the α-Fe → γ’-Fe4N transformation. In the first area, a solution of α-Fe (N) is formed with a continuous and insignificant change of the crystal lattice parameters of the iron lattice. In the second area, there is a step, oscillatory change in the parameters of the iron crystal lattice in FexN (x = 0.15, 0.20, 0.25 mol/mol). In the range of γ’-Fe4N → ε-Fe3-2N transformation, a solution is formed, with nitrogen concentration varying from 0.25–0.45 mol/mol. During the final stage of the nitriding process, at a constant value of the relative magnetic permeability, only the concentration of nitrogen in the solution εr increases. The rate of the phenomenon studied is limited by a diffusion rate through the top layer of atoms on the surface of iron nanocrystallite. The estimated value of the nitrogen diffusion coefficient varied exponentially with the degree of nitriding. In the area of the solution, the diffusion coefficient is approximately constant and amounts to 5 nm2/s. In the area of oscillatory changes, the average diffusion coefficient changes in the range of 3–11 nm2/s, and is inversely proportional to the nitrogen content degree. The advantage of the research method proposed in this paper is the possibility of simultaneously recording, under reaction conditions, changes in the values of several process parameters necessary to describe the process. The research results obtained in this way can be used to develop such fields of knowledge as heterogeneous catalysis, materials engineering, sensorics, etc.

XPS study of the nitridation of hafnia on silicon

We report a comprehensive quantitative ARXPS study of 20-cycle ALD-grown HfO2 films on Si (001) substrates with and without remote plasma nitridation. The films comprised a 0.6 nm HfO2 layer atop a 1 nm hafnium-rich silicate interlayer. A detailed analysis of the Hf 4f, Si 2p, O 1s, C 1s, and N 1s core level spectra is presented for samples with and without nitridation. The fitting of the Hf 4f spectra reveals four contributing components associated with silicate, hafnia, and their respective nitrided species. The Hf 4f, O 1s, and C 1s spectra exhibit a negative binding energy shift of about 0.2 eV upon nitridation induced by a Si2+-N dipole formed at the interface. Quantitative ARXPS analysis, employing the MultiLayer Model, enabled the determination of the thicknesses, and composition together with their associated uncertainties. The analysis showed that the composition of the nitrided hafnia is HfO1.66N0.33. There is a higher degree of nitridation in the silicate interlayer compared to the overlying hafnia layer. The degree of nitridation is found to increase with plasma power; N2+ is identified as the primary plasma species responsible for the nitridation of hafnia.

Surface Nitriding Enables Improved Intercalation Pseudocapacitance of T−Nb2O5 for Lithium‐Ion Batteries

AbstractThe low electrical conductivity limits the application of T−Nb2O5 as anode materials for practical applications. The large‐grain T−Nb2O5 nanowires with nitrogen doping are obtained by simple hydrothermal and annealing treatments. After nitriding with ammonia, an amorphous layer of NbON as an intermediate is formed and then the new crystalline NbN is generated with the deepening of the nitriding degree, causing homogeneous pores on the surface of the T−Nb2O5 and thus increasing the specific surface area. Meanwhile, the formation of the nitriding layer enhances the electrical conductivity, inducing a pseudo‐capacitance mechanism. Therefore, the N−T−Nb2O5 nanowire demonstrates superior electrochemical performance. Specifically, the 45N−T−Nb2O5 nanowires delivered a first reversible specific capacity of 238.33 mAh g−1 at 0.1 C, higher than of theoretical capacity (201.6 mAh g−1). However, the excessive nitride will be completely converted into NbN, reducing the initial lithium storage of the material due to the lack of a lithium storage site, which has not been previously discussed. Besides, at the high rate of 100 C, with the deepening of nitride degree of the T−Nb2O5, the higher the rate performance. Therefore, to find the balance between the high rate performance and the initial lithium storage amount, the nitride degree should be strictly controlled.

Quantitative Analysis of Degree of Nitridation of Ta2O5 to TaON Particles and their Photocatalytic Oxygen Evolution Properties

AbstractOxynitride or nitride photocatalysts are typically synthesized by heating oxide precursor particles at high temperatures under NH3 flow. Although there are numerous reports on optimizing the nitridation conditions for different materials, the detailed nitridation mechanism is still unclear. In this study, the nitridation of Ta2O5 to TaON was investigated. The degree of nitridation was quantitatively analyzed using the change in the weight of a powder after nitridation. Three regions were identified in the nitridation process for different nitridation times and NH3 flow rates. Moreover, the initial stage of the nitridation process (linear region) was formulated for the first time. Structural analysis, absorption properties, and oxygen evolution activity were combined with the degree of nitridation to reveal the relationship between the nitridation process and photocatalytic activity. The results of this work are useful for understanding and improving the photocatalytic properties of oxynitrides.

Low-temperature synthesis of high-purity AlN from Al powder

This study developed and optimized a novel and cost-effective method for producing AlN particles by heating Al powder in a nitrogen atmosphere at temperatures lower than the melting point of Al. As the size of the initial Al particles decreased, nitridation was accelerated because of the high specific surface area (more sites to initiate nitridation) and porosity (more pathways for nitrogen gas). The nitridation rate was initially slow for larger Al particles because of the time required to achieve a sufficiently low oxygen content, whereas almost complete nitridation of 1 μm Al powder rapidly occurred. The nitridation degree was <50% for 10 μm Al powder, while high-purity AlN particles (>95% nitridation) were obtained from the smaller Al particles. Process optimization demonstrated efficient and industrially relevant strategies for producing AlN from Al particles. The use of a mixture of different Al particle sizes greatly increased the degree of nitridation compared to the use of only 10 μm Al particles because of the high specific surface area and limited coarsening of Al by limiting the molten Al phase. The addition of Ar gas regulates the nitridation rate to avoid exhausting the nitrogen supply, which results in a very high degree of nitridation (98.7%). These findings demonstrate the production of high-purity AlN from Al powder at much lower temperatures than those currently used for commercial AlN fabrication. This could significantly reduce the manufacturing cost and promote wider use of AlN to take advantage of its attractive properties for electronics and other applications.

Developing First Row Transition Metal Antimonate Oxynitride and Oxysulfide Nanoparticles As Oxygen Reduction Electrocatalysts

Hydrogen fuel cells (FCs) are a promising avenue for replacing global dependence on carbon-intensive fuels especially in the transportation sector. The cathode of a FC carries out the oxygen reduction reaction (ORR), and the kinetics of the ORR is one of the primary sources of inefficiency in commercial FC devices. Commercial FC cathodes are based on expensive platinum-based catalyst architectures owing to their high activity, and therefore identifying highly active, non-precious ORR catalysts could accelerate the deployment of hydrogen fuel cells into the energy landscape. Theory calculations and experiments indicate first-row transition metal antimonates (MSbOx, M = Mn, Fe, Cr, Ni) have highly desirable ORR activity and stability characteristics. Furthermore, the material systems in this family have been extended to ternary and quaternary compositions for the ORR through the inclusion of additional transition metals to identify further opportunities for activity enhancement. In this work, we develop (oxy)sulfides and (oxy)nitrides of first-row transition metals such as manganese (MnSbOxSy and MnSbOxNz, respectively) to extend this class of materials towards enhanced activity when compared to previously reported antimonate materials. These nanoscale electrocatalysts are synthesized via colloidal means followed by high temperature surface modification under various reactive atmospheres (NH3, H2S, O2). Precise control of temperature ramps and holds enables modulation of the degree of oxidation, sulfidation, or nitridation, resulting in fully oxidized manganese antimonate nanocrystal cores with thin polycrystalline shells of (oxy)sulfide and (oxy)nitride. We tune the synthetic parameters of these materials and evaluate their enhanced electrochemical ORR activity in both alkaline and acidic conditions that are relevant to the operating conditions of anion-exchange membrane and proton-exchange membrane FCs, respectively. We perform extensive bulk and surface characterization with techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and gas adsorption BET surface area analysis. We find that converting the pure oxide into oxysulfide or oxynitride causes significant alterations to crystallinity and is accompanied by the appearance of additional crystalline phases that were not present previously as seen in XRD diffractograms. Transmission electron micrographs reveal polycrystalline nanocrystals of roughly 50 nm diameter for all compositions and show varying internal contrast patterns. The pure oxide nanocrystals form a thin shell of ~4 nm thickness after sulfidation or nitridation, and this shell may be converting into an active phase exposed to electrolyte during ORR testing and is expected to be significant for understanding the observed activity enhancement. XPS spectra indicate shifts in Sb and Mn peaks that are suggestive of changing surface-level oxidation state. We also utilize nanometer-resolution elemental mapping with TEM coupled to energy dispersive X-ray spectroscopy (EDS) to visualize the distribution of elements both before and after catalysis to understand how degradation affected observed activity trends. Lastly, on-line degradation measurements during potential cycling on an electrochemical flow cell coupled to inductively coupled plasma-mass spectrometry (ICP-MS) reveal potential and current density dependence of corrosion to provide insight into degradation mechanisms. Utilization of a diverse set of physical and electrochemical characterization techniques provides a thorough description of how manganese antimonate (oxy)sulfides and (oxy)nitrides carry out the ORR and could inform the future development of advanced non-precious FC cathode catalysts to drive down device cost.

Enhanced thermal conductivity and flexural strength of sintered reaction‐bonded silicon nitride with addition of (Y0.96Eu0.04)2O3

AbstractSintered reaction‐bonded silicon nitride (SRBSN) with high thermal conductivity was obtained using (Y0.96Eu0.04)2O3 and MgO as sintering additives. Green compacts were nitrided at 1400°C for 4 h. Post‐sintering was carried out at 1850 and 1900°C for 4 h, respectively. In reaction‐bonded silicon nitride (RBSN) doped with Y2O3 and MgO, the β‐Si3N4 content and nitridation degree were 51.1% and 93.8%, respectively. However, the β‐Si3N4 content and nitridation degree were 72.6% and 96.7% in a nitrided compact doped with (Y0.96Eu0.04)2O3 and MgO. After post‐sintering, the phase composition, microstructure, mechanical properties, and thermal conductivity were investigated. After sintering at 1900°C for 4 h, the thermal conductivity of SRBSN doped with (Y0.96Eu0.04)2O3 and MgO was increased by 16.5% compared to that of the samples doped with Y2O3 and MgO. The highest hardness of 1639 HV and the good flexural strength of 776.4 MPa were also achieved in the sample doped with 2‐mol.% (Y0.96Eu0.04)2O3 and 5‐mol.% MgO.

Guidelines for the synthesis of molybdenum nitride: Understanding the mechanism and the control of crystallographic phase and nitrogen content

Bulk (Mo2N) and supported molybdenum nitride (Mo2N/TiO2, Mo2N/CeO2) were synthesized by reduction-nitridation of bulk and supported MoO3 under a mixture of N2/H2. We investigated the mechanism of formation of molybdenum nitrides by coupling elemental analysis, in situ and ex situ XRD, Raman spectroscopy, in situ XPS/UPS spectroscopy and DFT calculation. We showed that it is possible to control the crystallographic phase and degree of nitridation (i.e. nitrogen vacancies) by adjusting the parameters of the synthesis, including the heating rate and final temperature, the nature, composition and flow of gas during reduction/nitridation and cooling. We demonstrated that β-Mo2N and γ-Mo2N go through the same intermediates: molybdenum bronze, MoO2 and Mo. The final crystallographic phase of the nitride depends mainly on the temperature rate and gas hourly space velocity employed during the synthesis, as they impact the crystallite size of the intermediates and the kinetics of reduction and nitridation. Molybdenum nitride clusters (<0.5 nm) are formed on TiO2.

Even Incorporation of Nitrogen into Fe0 Nanoparticles as Crystalline Fe4N for Efficient and Selective Trichloroethylene Degradation.

Surface modification of microscale Fe powder with nitrogen has emerged recently to improve the reactivity of Fe0 for dechlorination. However, it is unclear how an even incorporation of a crystalline iron nitride phase into Fe0 nanoparticles affects their physicochemical properties and performance, or if Fe0 nanoparticles with a varied nitridation degree will act differently. Here, we synthesized nitridated Fe0 nanoparticles with an even distribution of N via a sol-gel and pyrolysis method. Nitridation expanded the Fe0 lattice and provided the Fe4N species, making the materials more hydrophobic and accelerating the electron transfer, compared to un-nitridated Fe0. These properties well explain their reactivity and selectivity toward trichloroethylene (TCE). The TCE degradation rate by nitridated Fe0 (up to 4.8 × 10-2 L m-2 h-1) was much higher (up to 27-fold) than that by un-nitridated Fe0, depending on the nitridation degree. The materials maintained a high electron efficiency (87-95%) due to the greatly suppressed water reactivity (109-127 times lower than un-nitridated Fe0). Acetylene was accumulated as the major product of TCE dechlorination via β-elimination. These findings suggest that the nitridation of Fe0 nanoparticles can change the materials' physicochemical properties, providing high reactivity and selectivity toward chlorinated contaminants for in situ groundwater remediation.

CoNiO2 /Co4 N Heterostructure Nanowires Assisted Polysulfide Reaction Kinetics for Improved Lithium-Sulfur Batteries.

The “shuttle effect” of soluble polysulfides and slow reaction kinetics hinder the practical application of Li–S batteries. Transition metal oxides are promising mediators to alleviate these problems, but the poor electrical conductivity limits their further development. Herein, the homogeneous CoNiO2/Co4N nanowires have been fabricated and employed as additive of graphene based sulfur cathode. Through optimizing the nitriding degree, the continuous heterostructure interface can be obtained, accompanied by effective adjustment of energy band structure. By combining the strong adsorptive and catalytic properties of CoNiO2 and electrical conductivity of Co4N, the in situ formed CoNiO2/Co4N heterostructure reveals a synergistic enhancement effect. Theoretical calculation and experimental design show that it can not only significantly inhibit “shuttle effect” through chemisorption and catalytic conversion of polysulfides, but also improve the transport rate of ions and electrons. Thus, the graphene composite sulfur cathode supported by these CoNiO2/Co4N nanowires exhibits improved sulfur species reaction kinetics. The corresponding cell provides a high rate capacity of 688 mAh g−1 at 4 C with an ultralow decaying rate of ≈0.07% per cycle over 600 cycles. The design of heterostructure nanowires and graphene composite structure provides an advanced strategy for the rapid capture–diffusion–conversion process of polysulfides.

Characteristics of High Surface Area Molybdenum Nitride and Its Activity for the Catalytic Decomposition of Ammonia

High surface area (&gt;170 m2 g−1) molybdenum nitride was prepared by the temperature-programmed nitridation of α-MoO3 with pure ammonia. The process was optimized by adjusting the experimental variables: the reaction temperature, heating rate, and molar flow rate of ammonia. The physicochemical properties of the as-formed molybdenum nitride were characterized by X-ray diffraction, N2 sorption, transmission electron microscopy, temperature-programmed oxidation/reduction, and X-ray photoelectron spectroscopy. Of the experimental variables, the nitridation temperature was found to be the most critical parameter determining the surface area of the molybdenum nitride. When the prepared molybdenum nitride was exposed to air, the specific surface area rapidly decreased because of the partial oxidation of molybdenum nitride to molybdenum oxynitride. However, the surface area recovered to 90% the initial value after H2 treatment. The catalyst with the highest degree of nitridation showed the best catalytic activity, superior to that of unmodified α-MoO3, for the decomposition of ammonia because of its high surface area.

Three Orders of Reverse Leakage Reduction by Using Supercritical CO2 Nitriding Process on GaN Quasi-Vertical Schottky Barrier Diode

Surface treatment is quite vital to reduce the reverse leakage current and improve the current On-Off ratio of gallium nitride (GaN) quasi-vertical Schottky barrier diode (SBD). In this work, we report a new nondestructive low temperature supercritical CO2 fluid (SCF) nitridation technology to treat the surface of GaN and substantially reduce the leakage current of GaN quasi-vertical SBD. The X-ray photoelectron spectroscopy (XPS) reveals the increased nitriding degree and oxygen doping degree in the form of GaOx after the SCF process results in the leakage performance improvement. As a result, the defects at metal/semiconductor interface are passivated. Also, because of NH-2 and OH- generated from the SCF process, the surface peak electric field of n-GaN decreases, and thus the breakdown voltage is enhanced. The reverse leakage current density is reduced by more than three orders of magnitude from 3.7 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> to 3.52 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (at -3 V) and the ON-/OFF-current ratio is improved by three orders from 8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> to 5.74 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> .

Chromium nitride synthesis in co-flow filtration mode

In this work, we considered an original setup that allows us to study the combustion of metal powders in a co-flow gas stream, and also presents the results of experimental studies on the combustion of chromium powder in a confined nitrogen-containing gas stream. This method has high energy efficiency and allows to get nitrides without the use of high pressures. The study of chromium combustion is justified by its high melting point at a relatively low exothermicity, which eliminates the influence of melting of the initial powder and synthesis products on the filtration process. This article presents the results of combustion of chromium powder with particle size δ = 63÷80 µm and δ = 500÷1000 μm. X-ray phase studies of the obtained powders were carried out. The effect of nitrogen-containing gas consumption on the degree of nitriding of combustion products is shown. It is established that the forced filtration mode allows the synthesis of non-stoichiometric chromium nitride Cr2N. The nitrogen content in the obtained Cr2N samples reached 9.6%.

Effect of Material and Process Variables on Characteristics of Nitridation-Induced Self-Formed Aluminum Matrix Composites-Part 1: Effect of Reinforcement Volume Fraction, Size, and Processing Temperatures.

This paper investigates the effect of the size and volume fraction of SiC, along with that of the processing temperature, upon the nitridation behavior of aluminum powder during the nitridation-induced self-formed aluminum composite (NISFAC) process. In this new composite manufacturing process, aluminum powder and ceramic reinforcement mixtures are heated in nitrogen gas, thus allowing the exothermic nitridation reaction to partially melt the aluminum powder in order to assist the composite densification and improve the wetting between the aluminum and the ceramic. The formation of a sufficient amount of molten aluminum is key to producing sound, pore-free aluminum matrix composites (AMCs); hence, the degree of nitridation is a key factor. It was demonstrated that the degree of nitridation increases with decreasing SiC particle size and increasing SiC volume fraction, thus suggesting that the SiC surface may act as an effective pathway for nitrogen gas diffusion. Furthermore, it was found that effective nitridation occurs only at an optimal processing temperature. When the degree of nitridation is insufficient, molten Al is unable to fill the voids in the powder bed, leading to the formation of low-quality composites with high porosities. However, excessive nitridation is found to rapidly consume the nitrogen gas, leading to a rapid drop in the pressure in the crucible and exposing the remaining aluminum powder in the upper part of the powder bed. The nitridation behavior is not affected by these variables acting independently; therefore, a systematic study is needed in order to examine the concerted effect of these variables so as to determine the optimal conditions to produce AMCs with desirable properties for target applications.

Effect of Material and Process Variables on Characteristics of Nitridation-Induced Self-Formed Aluminum Matrix Composites-Part 2: Effect of Nitrogen Flow Rates and Processing Temperatures.

The nitridation-induced self-formed aluminum matrix composite (NISFAC) process is based on the nitridation reaction, which can be significantly influenced by the characteristics of the starting materials (e.g., the chemical composition of the aluminum powder and the type, size, and volume fraction of the ceramic reinforcement) and the processing variables (e.g., process temperature and time, and flow rate of nitrogen gas). Since these variables do not independently affect the nitridation behavior, a systematic study is necessary to examine the combined effect of these variables upon nitridation. In this second part of our two-part report, we examine the effect of nitrogen flow rates and processing temperatures upon the degree of nitridation which, in turn, determines the amount of exothermic reaction and the amount of molten Al in the nitridation-induced self-formed aluminum matrix composite (NISFAC) process. When either the nitrogen flow rate or the set temperature was too low, high-quality composites were not obtained because the level of nitridation was insufficient to fill the powder voids with molten Al. Hence, since the filling of the voids in the powder bed by molten Al is essential to the NISFAC process, the conditions should be optimized by manipulating the nitrogen flow rate and processing temperature.

Combustion of Ferrotitanium in Nitrogen

Combustion of FTi70 industrial ferrotitanium containing 65.9% Ti and a model alloy of iron with 30.6% Ti in nitrogen is described. It is shown that the degree of nitriding of the combustion products of industrial ferrotitanium is low. The distribution of nitrogen along the sample diameter is uneven. The dilution of the alloy with the final product increases in the degree of nitriding. The study of the combustion of the model alloy concludes that the degree of nitriding increases along with the melting point of the initial alloy. The composition of the combustion products of ferrotitanium is described.

Structural, magnetic and electronic properties of EuTi0.5W0.5O3-xNx perovskite oxynitrides

EuTi0.5W0.5O3-xNx oxynitrides with nitrogen contents between 0.87 and 1.63 have been synthesized by ammonolysis of EuTi0.5W0.5O4 scheelite at high temperature. They are B-site disordered perovskites that show two different crystal symmetries as a function of the nitrogen content, changing at x near 1.3 from cubic Pm-3m to orthorhombic Pbnm. The nitriding degree is tuned by the ammonia flow rate and synthesis temperature, and determines the oxidation states of the cations and the equilibrium Eu2+ ​+ ​W6+ ↔ Eu3++W5+ promoting the decrease of the tolerance factor and the structural transition for large x values and electronic reconstructions. Accompanying the symmetry lowering, the nature of the magnetic order of Eu2+ moments changes from ferromagnetic to antiferromagnetic as a result of a complex interplay of antiferromagnetic/ferromagnetic superexchange interactions balanced by structural effects and the carrier-mediated magnetic exchange between Eu2+ ions.

Effects of nitriding on the microstructure and properties of VAlTiCrMo high-entropy alloy coatings by sputtering technique

The VAlTiCrMo and (VAlTiCrMo)Nx coatings were prepared by sputtering V–Al–Ti–Cr–Mo splicing targets. The microstructure and chemical composition were analyzed by means of X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and transmission electron microscope (TEM). The high-entropy alloy coating and the corresponding nitride coating have body-centered cubic (BCC) and face-centered cubic (FCC) structures respectively. Nitriding changes the phase structure and growth mode of the high-entropy coating, and the synthesized high-entropy nitride coatings are more compact. Meanwhile, the introduction of nitrogen leads to severe lattice distortion, which improves the mechanical properties of the high-entropy coating. As the nitriding degree increases, the corrosion resistance of the high-entropy nitride coatings in 3.5 wt% NaCl solution is improved. The synthesized (VAlTiCrMo)Nx coating exhibits optimized mechanical properties and nobler corrosion resistance when the N2 flow is 800 sccm.

Fabrication and characterisation of AlN-SiC porous composite ceramics by nitridation of Al4SiC4

AlN-SiC porous composite ceramics were fabricated by heat-treating Al4SiC4 porous ceramic green body prepared using the foaming method under flowing nitrogen. The porous structure could provide increased nitrogen diffusion channels, leading to a decrease in N2 diffusion resistance and an increasing degree of nitridation. The micro-morphology of AlN was in the form of particles or whiskers and influenced by heating temperature and time parameters. AlN has good dispersibility with SiC particles, resulting in local homogeneous chemical composition of prepared AlN-SiC composite ceramics. Furthermore, the potential reaction mechanism responsible for the synthesis of AlN-SiC porous composite ceramics during the nitridation was revealed. This simple approach may be used for large-scale synthesis of AlN-SiC porous composite ceramics.

Formation Manner of Lattice Defect in BaNbO2n Photocatalysts during Their Nitridation in Molten Flux

Nowadays, development of energy conversion from natural sources is essential issue for construction of sustainable society and photocatalytic water oxidation using sun light is one of desirable processes to achieve it. For effective use of sun light, it is necessary to utilize visible light, since the solar provides over 50 % of visible light, but only 5 % of UV light in its whole energy. (Oxy)nitrides are promising candidates for visible-light response photocatalysts. BaNbO2N is one of oxynitrides as photocatalyst, which belongs to cubic-type perovskite. BaNbO2N exhibits superior visible-light absorption up to ca. 740 nm (band gap energy: ca. 1.7 eV) compared to other (oxy)nitrides: For example, the absorption edges are 700 nm for SrNbO2N, 650 nm for LaTaO2N and BaTaO2N, and 600 nm for LaTiO2N. Previous reports about BaNbO2N indicated that the synthetic conditions affect to the photocatalytic performance, while there is still plenty of space to fully understand what kind of factors exist and how much they contribute to the performance. Lattice defects, such as atomic vacancy and electric defect, should be one of dominant factors to determine the photocatalytic performance. It is known that they exhibit two kinds of aspects in photocatalytic reaction : The first one is recombination center of excited electron, and second one is modification of surficial electric states, which give complicated contributions to the performance. So, optimal design of lattice defects based on well understanding of their structure and role to photocatalytic performance is necessary to improve the photocatalytic performance of BaNbO2N up to theoretical value. BaNbO2N is usually prepared under reduction condition, which leads reduction of niobium (V) and formation of lattice defects. Considering that thermodynamic parameters correlate to formation of lattice defect, we expected that inhibition of the lattice defects is possible if optimal reaction pathway is adopted. So, we set our final goal of this research to discover reaction process to control the lattice defects down to negligible order. As a first step of our research, it is important to understand the origin of the lattice defects. In this presentation, we will report formation manner of the lattice defect in terms of their species, quantity, and location. Submicron, mono-dispersed BaNbO2N crystals with different nitridation degree were grown using flux method. Elemental, structural, and valence analyses were performed to determine chemical formula with lattice defects in each nitridation-state sample to describe chemical states of BaNbO2N in each nitridation degree. We also introduced mathematic statistics analyses to visualize correlation between lattice defect formations and their driving force, which should give hint to suggest optimal preparation pathway to non-lattice defect BaNbO2N. Acknowledgement : This work was supported by JSPS Grant-in-Aid for Scientific Research (A) 25249089, Regional Innovation and Ecosystem Formation Program from the MEXT, and JST Program on Open Innovation Platform with Enterprises, Research Institute and Academia.